On an actual website we’re building (gasp) in Rails, we have a bunch of content types that have promotions.

Here’s the schema for our promotions table:

create_table :promotions do |t|
  t.column :size,       :integer
  t.column :content,    :text
  t.column :item_type,  :string
  t.column :item_id,    :integer
  t.column :created_at, :datetime
end

And the the association code to wire up the promotions:

class Story < ActiveRecord::Base
  has_many :promotions, :conditions => ["item_type = ?", self.class.to_s], 
                        :foreign_key => "item_id" 
end

class Product < ActiveRecord::Base
  has_many :promotions, :conditions => ["item_type = ?", self.class.to_s], 
                        :foreign_key => "item_id" 
end

Which gives us the ability to do story.promotions or product.promotions and the model code is exactly the same. It’s straightforward without any unnecessary fuss, but Rails does provide some magic to make it even cleaner:

class Story < ActiveRecord::Base
  has_many :promotions, :as => :item
end

class Product < ActiveRecord::Base
  has_many :promotions, :as => :item
end

Yes, I’ll admit, for all of the shit I give Rails it does do some pretty cool stuff.